Carbonaceous composite heat spreader and associated methods

ABSTRACT

A heat spreader including a plurality of carbonaceous particles present in an amount of at least about 50% by volume of the heat spreader. A non-carbonaceous material is also present in an amount of at least about 5% by volume of the heat spreader, the non-carbonaceous material including an element selected from the group consisting of Cu, Al and Ag. In another aspect, the carbonaceous particles may be sintered or fused directly to one another. The heat spreader can be incorporated into a cooling unit for transferring heat away from a heat source, which includes a heat sink with the heat spreader disposed in thermal communication with both the heat sink and the heat source.

PRIORITY INFORMATION

This application is a continuation of copending U.S. patent applicationSer. No. 10/453,469, filed Jun. 2, 2003, which is a continuation-in-partof U.S. patent application Ser. No. 10/270,018, filed Oct. 11, 2002,which are each herein incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to carbonaceous composite devices andsystems that can be used to conduct or absorb heat away from a heatsource. Accordingly, the present invention involves the fields ofchemistry, physics, and materials science.

BACKGROUND OF THE INVENTION

Progress in the semiconductor industry has been following the trend ofMoore's Law that was proposed in 1965 by then Intel's cofounder GordonMoore. This trend requires that the capability of integrated circuits(IC) or, in general, semiconductor chips double every 18 months. Thus,the number of transistors on a central processing unit (CPU) in 2002 mayapproach 100 million. As a result of this densification of circuitry,line-width in 2002 narrowed to 0.18 micrometer and more advanced chipsare using wires as thin as 0.13 micrometer. With this trend continuing,it is projected that the seemingly impermeable “Point One” barrier, of0.1 micrometer, will be attained and surpassed in the next few years.

Along with such advances comes various design challenges. One of theoften overlooked challenges is that of heat dissipation. Most often,this phase of design is neglected or added as a last minute designbefore the components are produced. According to the second law ofthermodynamics, the more work that is performed in a closed system, thehigher entropy it will attain. With the increasing power of a CPU, thelarger flow of electrons produces a greater amount of heat. Therefore,in order to prevent the circuitry from shorting or burning out, the heatresulting from the increase in entropy must be removed. Somestate-of-the-art CPUs have a power of about 60 watts (W). For example, aCPU made with 0.13 micrometer technology may exceed 100 watts. Currentmethods of heat dissipation, such as by using metal (e.g., Al or Cu) finradiators, and water evaporation heat pipes, will be inadequate tosufficiently cool future generations of CPUs.

Recently, ceramic heat spreaders (e.g., AlN) and metal matrix compositeheat spreaders (e.g., SiC/Al) have been used to cope with the increasingamounts of heat generation. However, such materials have a thermalconductivity that is no greater than that of Cu, hence, their ability todissipate heat from semiconductor chips is limited.

A typical semiconductor chip contains closely packed metal conductors(e.g., Al, Cu) and ceramic insulators (e.g., oxide, nitride). Thethermal expansion of metal is typically 5-10 times that of ceramics.When the chip is heated to above 60° C., the mismatch of thermalexpansions between metal and ceramics can create microcracks. Therepeated cycling of temperature tends to aggravate the damage to thechip. As a result, the performance of the semiconductor willdeteriorate. Moreover, when temperatures reach more than 90° C., thesemiconductor portion of the chip may become a conductor so the functionof the chip is lost. In addition, the circuitry may be damaged and thesemiconductor is no longer usable (i.e. becomes “burned out”). Thus, inorder to maintain the performance of the semiconductor, its temperaturemust be kept below a threshold level (e.g., 90° C.).

A conventional method of heat dissipation is to contact thesemiconductor with a metal heat sink. A typical heat sink is made ofaluminum that contains radiating fins. These fins are attached to a fan.Heat from the chip will flow to the aluminum base and will betransmitted to the radiating fins and carried away by the circulated airvia convection. Heat sinks are therefore often designed to have a highheat capacity to act as a reservoir to remove heat from the heat source.

Alternatively, a heat pipe may be connected between the heat sink and aradiator that is located in a separated location. The heat pipe containswater vapor that is sealed in a vacuum tube. The moisture will bevaporized at the heat sink and condensed at the radiator. The condensedwater will flow back to the heat sink by the wick action of a porousmedium (e.g., copper powder). Hence, the heat of a semiconductor chip iscarried away by evaporating water and removed at the radiator bycondensing water.

Although heat pipes and heat plates may remove heat very efficiently,the complex vacuum chambers and sophisticated capillary systems preventdesigns small enough to dissipate heat directly from a semiconductorcomponent. As a result, these methods are generally limited totransferring heat from a larger heat source, e.g., a heat sink. Thus,removing heat via conduction from an electronic component is acontinuing area of research in the industry.

One promising alternative that has been explored for use in heatspreaders is diamond-containing materials. Diamond can carry away heatmuch faster than any other material. The thermal conductivity of diamondat room temperature (about 2000 W/mK) is much higher than either copper(about 400 W/mK) or aluminum (250 W/mK), the two fastest metal heatconductors commonly used. Moreover, the thermal capacity of diamond (1.5J/cm³) is much lower than copper (17 J/cm³) or aluminum (24 J/cm³). Theability for diamond to carry away heat without storing it makes diamondan ideal heat spreader. In contrast to heat sinks, a heat spreader actsto quickly conduct heat away from the heat source without storing it.Table 1 shows various thermal properties of several materials ascompared to diamond (values provided at 300 K). TABLE 1 Thermal ThermalConductivity Heat Capacity Expansion Material (W/mK) (J/cm³ K) (1/K)Copper 401 3.44 1.64E−5 Aluminum 237 2.44  2.4E−5 Molybdenum 138 2.574.75E−5 Gold 317 2.49 1.43E−5 Silver 429 2.47 1.87E−5 Tungsten Carbide95 2.95 0.57E−5 Silicon 148 1.66 0.258E−5  Diamond (IIa) 2,300 1.780.14E−5

In addition, the thermal expansion coefficient of diamond is one of thelowest of all materials. The low thermal expansion of diamond makesjoining it with low thermally expanding silicon semiconductor mucheasier. Hence, the stress at the joining interface can be minimized. Theresult is a stable bond between diamond and silicon that does notdelaminate under the repeated heating cycles.

In recent years diamond heat spreaders have been used to dissipate heatfrom high power laser diodes, such as that used by laser diodes to boostthe light energy in optical fibers. However, large area diamonds arevery expensive; hence, diamond has not been commercially used to spreadthe heat generated by CPUs. In order for diamond to be used as a heatspreader, its surface must be polished so it can make an intimatecontact with the semiconductor chip. Moreover, its surface may bemetallized (e.g., by Ti/Pt/Au) to allow attachment to a conventionalmetal heat sink by brazing.

Many current diamond heat spreaders are made of diamond films formed bychemical vapor deposition (CVD). One example of raw CVD diamond filmsare now sold at over $10/cm², and this price may doubled when it ispolished and metallized. This high price would prohibit diamond heatspreaders from being widely used except in those applications (e.g.,high power laser diodes) where only a small area is required or noeffective alternative heat spreaders are available. In addition to beingexpensive, CVD diamond films can only be grown at very slow rates (e.g.,a few micrometers per hour); hence, these films seldom exceed athickness of 1 mm (typically 0.3-0.5 mm). However, if the heating areaof the chip is large (e.g., a CPU), it is preferable to have a thicker(e.g., 3 mm) heat spreader.

In addition to diamond products produced using CVD methods, attemptshave been made to form heat spreaders using a mass of particulatediamond or “polycrystalline diamond” (PCD). Specific examples of suchdevices are found in U.S. Pat. No. 6,390,181, and U.S. PatentApplication Publication No.2002/0023733, each of which is incorporatedherein by reference. Typically, a PCD product (or “compact”) is formedby processing diamond particles under high-pressure, high-temperature(HPHT) conditions to thereby cause the diamond particles to sinter orbond to each other and/or an interstitial material. As a result, mostPCD compacts have a relatively small thickness due to the extremepressures required by the HPHT process. Because of the extremely highpressures used, the mold or cavity used in producing PCD compacts hasbeen limited to a small thickness so that the mechanical equipmentcreating the extremely high pressure can maintain the pressure andtemperature required. Such PCD compacts are of limited use in the fieldof heat spreaders because of their limited physical capacity to transferor conduct heat.

As such, cost effective systems and devices that are capable ofeffectively conducting heat away from a heat source, continue to besought through ongoing research and development efforts.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides composite heat spreadersthat can be used to draw or conduct heat away from a heat source. In oneaspect, the heat spreader includes a plurality of carbonaceous particlespresent in an amount of at least about 50% by volume of the heatspreader. A non-carbonaceous infiltrant can be present in an amount ofat least about 5% by volume of the heat spreader. The non-carbonaceousinfiltrant can include an element selected from the group consisting ofCu, Al and Ag.

In accordance with another aspect of the invention, the carbonaceousparticles can be present in an amount of at least about 80% by volume ofthe heat spreader, and in another aspect can be present in an amount ofat least about 90% by volume of the heat spreader. The carbonaceousparticles can comprise diamond particles.

In accordance with another aspect of the invention, the infiltrantincludes a carbide forming element and from about 1% w/w to about 10%w/w of either Cu, Al, or Ag. The carbide forming element can be a memberselected from the group consisting of: Sc, Y, Ti, Zr, Hf, V, Nb, Cr, Mo,Mn, Ta, W, Tc, Si, B, Al, and alloys thereof. The carbide formingelement can also be a Cu—Mn alloy.

In accordance with another aspect of the invention, the infiltrant canbe an alloy having a eutectic melting point below about 1100° C. Theinfiltrant can be an alloy that wets the carbonaceous particles.

In accordance with another aspect of the invention, the diamondparticles can be present in an amount of greater than about 50% byvolume of the heat spreader, and the infiltrant can be present in anamount of greater than about 5% by volume of the heat spreader, and cancontain at least about 2% w/w of a carbide former.

In accordance with yet another aspect of the invention, a cooling unitfor transferring heat away from a heat source is provided, and includesa heat sink and a heat spreader in accordance with the heat spreaderembodiments recited herein. The heat spreader can be disposed in thermalcommunication with both the heat sink and the heat source.

In accordance with yet another aspect of the invention, a cooling unitfor transferring heat away from a heat source is provided, and includesa heat sink and a heat spreader comprising a mass of diamond particlessintered directly to one another. The heat spreader can be in thermalcommunication with both the heat sink and the heat source.

In accordance with another aspect of the invention, the heat spreadercan be at least partially embedded in the heat source and/or the heatsink.

In accordance with another aspect of the invention, the heat spreader isheld in the heat sink by a compression fit. The compression fit holdingthe heat spreader in the heat sink can be a thermally inducedcompression fit.

In accordance with another aspect of the invention, the heat sinkcomprises a heat pipe having an internal working fluid. The heatspreader can protrude through a wall of the heat pipe, and can have abottom surface in direct contact with the working fluid of the heatpipe.

In accordance with still another aspect of the invention, a method ofmaking a heat spreader is provided and includes the steps of providing aplurality of carbonaceous particles, and infiltrating the plurality ofcarbonaceous particles with a non-carbonaceous infiltrant according toaspects of the heat spreaders as recited herein.

In accordance with yet another aspect of the invention, a method ofcooling a heat source is provided and includes the steps of providing aheat spreader according to aspects of heat spreaders recited herein, andplacing the heat spreader in thermal communication with both the heatsource and a heat sink.

There has thus been outlined, rather broadly, various features of theinvention so that the detailed description thereof that follows may bebetter understood, and so that the present contribution to the art maybe better appreciated. Other features of the present invention willbecome clearer from the following detailed description of the invention,taken with the accompanying claims, or may be learned by the practice ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a schematic view of a heat spreader in thermal communicationwith a heat source and a heat sink in accordance with an embodiment ofthe present invention;

FIG. 1 b is a schematic view of a heat spreader in thermal communicationwith a heat source and a heat sink in accordance with another embodimentof the present invention; and

FIG. 1 c is a schematic view of a heat spreader in thermal communicationwith a heat source and a heat sink in accordance with another embodimentof the present invention.

DETAILED DESCRIPTION

Before the present invention is disclosed and described, it is to beunderstood that this invention is not limited to the particularstructures, process steps, or materials disclosed herein, but isextended to equivalents thereof as would be recognized by thoseordinarily skilled in the relevant arts. It should also be understoodthat terminology employed herein is used for the purpose of describingparticular embodiments only and is not intended to be limiting.

It must be noted that, as used in this specification and the appendedclaims, the singular forms “a,” “an” and, “the” include plural referentsunless the context clearly dictates otherwise. Thus, for example,reference to “a diamond particle” includes one or more of suchparticles, reference to “an interstitial material” includes reference toone or more of such materials, and reference to “the particle” includesreference to one or more of such particles.

Definitions

In describing and claiming the present invention, the followingterminology will be used in accordance with the definitions set forthbelow.

As used herein, “particle” and “grit” may be used interchangeably, andwhen used in connection with a carbonaceous material, refer to aparticulate form of such material. Such particles or grits may take avariety of shapes, including round, oblong, square, euhedral, etc., aswell as a number of specific mesh sizes. As is known in the art, “mesh”refers to the number of holes per unit area as in the case of U.S.meshes. All mesh sizes referred to herein are U.S. mesh unless otherwiseindicated. Further, mesh sizes are generally understood to indicate anaverage mesh size of a given collection of particles since each particlewithin a particular “mesh size” may actually vary over a smalldistribution of sizes.

As used herein, “substantial,” or “substantially” refers to thefunctional achievement of a desired purpose, operation, orconfiguration, as though such purpose or configuration had actually beenattained. Therefore, carbonaceous particles that are substantially incontact with one another function as though, or nearly as though, theywere in actual contact with one another. In the same regard,carbonaceous particles that are of substantially the same size operate,or obtain a configuration as though they were each exactly the samesize, even though they may vary in size somewhat.

As used herein, “heat spreader” refers to a material which distributesor conducts heat and transfers heat away from a heat source. Heatspreaders are distinct from heat sinks which are used as a reservoir forheat to be held in, until it can be transferred away from the heat sinkby another mechanism, whereas a heat spreader does not retain asignificant amount of heat, but merely transfers heat away from a heatsource.

As used herein, “heat source” refers to a device or object having anamount of thermal energy or heat which is greater than desired. Heatsources can include devices that produce heat as a byproduct of theiroperation, as well as objects that become heated to a temperature thatis higher than desired by a transfer of heat thereto from another heatsource.

As used herein, “carbonaceous” refers to any material which is madeprimarily of carbon atoms. A variety of bonding arrangements or“allotropes” are known for carbon atoms, including planar, distortedtetrahedral, and tetrahedral bonding arrangements. As is known to thoseof ordinary skill in the art, such bonding arrangements determine thespecific resultant material, such as graphite, diamond-like-carbon(DLC), or amorphous diamond, and pure diamond. In one aspect, thecarbonaceous material may be diamond.

As used herein, “reactive element” and “reactive metal” may be usedinterchangeably, and refer to an element, especially a metal elementthat can chemically react with and chemically bond to carbon by forminga carbide bond. Examples of reactive elements may include withoutlimitation, transition metals such as titanium (Ti) and chromium (Cr),including refractory elements, such as zirconium (Zr) and tungsten (W),as well as non-transition metals and other materials, such as aluminum(Al). Further, certain non-metal elements such as silicon (Si) may beincluded as a reactive element in a brazing alloy.

As used herein “wetting” refers to the process of flowing a molten metalacross at least a portion of the surface of a carbonaceous particle.Wetting is often due, at least in part, to the surface tension of themolten metal, and may be facilitated by the use or addition of certainmetals to the molten metal. In some aspects, wetting may aid in theformation of chemical bonds between the carbonaceous particle and themolten metal at the interface thereof when a carbide forming metal isutilized.

As used herein, “chemical bond” and “chemical bonding” may be usedinterchangeably, and refer to a molecular bond that exerts an attractiveforce between atoms that is sufficiently strong to create a binary solidcompound at an interface between the atoms. Chemical bonds involved inthe present invention are typically carbides in the case of diamondsuperabrasive particles, or nitrides or borides in the case of cubicboron nitride.

As used herein, “braze alloy” and “brazing alloy” may be usedinterchangeably, and refer to an alloy containing a sufficient amount ofa reactive element to allow the formation of chemical bonds between thealloy and a superabrasive particle. The alloy may be either a solid orliquid solution of a metal carrier solvent having a reactive elementsolute therein. Moreover, the term “brazed” may be used to refer to theformation of chemical bonds between a superabrasive particle and a brazealloy.

As used herein, “sintering” refers to the joining of two or moreindividual particles to form a continuous solid mass. The process ofsintering involves the consolidation of particles to at least partiallyeliminate voids between particles. Sintering may occur in either metalor carbonaceous particles, such as diamond. Sintering of metal particlesoccurs at various temperatures depending on the composition of thematerial. Sintering of diamond particles generally requires ultrahighpressures and the presence of a carbon solvent as a diamond sinteringaid, and is discussed in more detail below. Sintering aids are oftenpresent to aid in the sintering process and a portion of such may remainin the final product.

Concentrations, amounts, particle sizes, volumes, and other numericaldata may be expressed or presented herein in a range format. It is to beunderstood that such a range format is used merely for convenience andbrevity and thus should be interpreted flexibly to include not only thenumerical values explicitly recited as the limits of the range, but alsoto include all the individual numerical values or sub-ranges encompassedwithin that range as if each numerical value and sub-range is explicitlyrecited.

As an illustration, a numerical range of “about 1 micrometer to about 5micrometers” should be interpreted to include not only the explicitlyrecited values of about 1 micrometer to about 5 micrometers, but alsoinclude individual values and sub-ranges within the indicated range.Thus, included in this numerical range are individual values such as 2,3, and 4 and sub-ranges such as from 1-3, from 2-4, and from 3-5, etc.This same principle applies to ranges reciting only one numerical value.Furthermore, such an interpretation should apply regardless of thebreadth of the range or the characteristics being described.

The Invention

The present invention encompasses devices, systems, and methods fortransferring heat away from a heat source. Heat spreaders made inaccordance with the method of the present invention generally contain aplurality of carbonaceous particles, including embodiments in whichcarbonaceous particles are each substantially in contact with oneanother. The plurality of carbonaceous particles may either be boundtogether using an interstitial material or by direct sintering or fusingof the carbonaceous particles themselves into a mass.

In one aspect, a general process for making a heat spreader having ahigh carbonaceous particle volume begins with the packing of a firstplurality of carbonaceous particles in a suitable mold. Optionally, thefirst plurality of particles may each be approximately the same meshsize. The specific size of these particles may be up to about 18 mesh (1mm) with sizes between about 30 mesh (0.5 mm) and about 400 mesh (37micrometers) being typical. While the size of the particles may vary,the general principle is recognized that larger carbonaceous particlesprovide for a larger path having improved heat transfer characteristicswhich approach that of a solid carbonaceous material, such as purediamond.

The particles are packed such that there is substantial contact betweenparticles. Each particle should be in contact with at least one otherparticle in the packed group. Thus, there may be groups of particleswhich are in contact with one another separate from the remainingparticles. In another aspect of the present invention the contactbetween particles may be sufficient to provide a continuous path tosubstantially all of the carbonaceous particles in the heat spreader.The transfer of heat away from the heat source is facilitated by thesubstantial particle-particle contact. The particles can be packed so asto occupy most of the volume and minimize the amount of empty voidbetween particles.

In one aspect, particle packing that obtains the above-recited goals maybe achieved by packing carbonaceous particles of different sizes insuccessive stages. For example, larger carbonaceous particles are packedinto a suitable mold. The packing of the carbonaceous particles may beimproved by settling or otherwise compacting, e.g., agitated inside themold by a vibrator. A plurality of smaller carbonaceous particles maythen be added to fill the voids surrounding the larger carbonaceousparticles. Depending on the size of the smaller particles, the smallerparticles may need to be introduced from multiple sides of the packedcarbon material in order to fill most of the available voids. The sizeof the smaller carbonaceous particles may vary. Typically particles inthe range of between about ⅓^(rd) to about 1/20^(th) of the diameter ofthe larger diamond will increase the packing efficiency. Particles whichare between about ⅕^(th) and about 1/10^(th) may also be used, whileparticles 1/7^(th) the diameter of the larger particles have been usedwith good results. Using such successive packing stages, the volumepacking efficiency may reach greater than two-thirds. If necessary,addition of even smaller carbonaceous particles may be performed toincrease the packing efficiency further. However, this successivepacking method will soon reach a point of diminishing returns as thefilling becomes more and more difficult while the increase in packingefficiency becomes less and less. The packed particles made inaccordance with the above principles in mind will provide a carbonvolume content of between about 50% and about 80%.

In an alternative embodiment, the different sized carbonaceous particlesare mixed first and then packed together prior to introduction of theinterstitial material. This approach allows for an increase in packingefficiency, however, some thermal benefits may be sacrificed as a resultof the larger particles not being in intimate contact with other largerparticles. Thus, heat must cross a greater number of particle-particleinterface boundaries increasing the thermal resistance of the final heatspreader.

In yet another alternative embodiment, the volume of diamond may beincreased by using uniformly shaped carbonaceous particles. Inparticular, substantially cubic diamond particles are commerciallyavailable, although other shapes could be used. The cubic diamonds maybe packed edge-to-edge to produce a layer, or layers, of packed diamondparticles with a diamond volume content of up to about 90%-95%. Thespecific arrangement is unimportant and the particles may be packed inordered rows and columns or the rows and columns may be staggered. Inthis embodiment, the arrangement of diamond particles allows forsubstantially smaller volume of void between particles without sinteringthe diamond particles together. In addition, the thermal properties ofthe final composite are improved if the particles are all oriented inthe same direction as opposed to random directions. The followingdiscussions of interstitial materials and processing apply to thisarrangement of packed diamond particles as to the above described packeddiamond particles.

In one aspect of the invention, an interstitial material may be used inconnection with the carbonaceous particles in order to bond themtogether into a composite mass. However, by packing the particles priorto introduction of the interstitial material as recited above, theoriginal particle-to-particle contact can be maintained so that thepacking efficiency far exceeds the efficiency obtained by first mixingthe carbonaceous particles with an interstitial material and thenconsolidating by hot pressing. In the latter case, the carbonaceousparticles are likely to constitute less than one-half of the devicevolume, as the interstitial material tends to fill around diamondparticles and between them, thus completely separating many particlesfrom one another. In this case, heat must cross significant areas ofnon-carbonaceous material.

Thus, in accordance with one aspect of the present invention, thecarbonaceous particles are packed before the introduction of anynon-carbonaceous materials as discussed above. One factor to consider indesigning a carbonaceous composite heat spreader of the presentinvention is the thermal properties of the composite at the interfacesbetween carbonaceous particles and the interfaces betweennon-carbonaceous material and carbonaceous particles. Empty voids andmere mechanical contact between interfaces act as a thermal barriers.Although intimate contact of carbonaceous particles along a significantportion of the surface of the particles improves the thermal propertiesat these boundaries, the result is somewhat inferior to that of purecontinuous carbonaceous material. Thus, it is desirable that asubstantial portion of the interfaces are more than mere mechanicalcontact.

Accordingly, an interstitial material may be utilized having specificcharacteristics suitable to achieving a particular finished tool. In oneaspect, the interstitial material may be suitable to act as a carbonsintering aid under ultrahigh pressure to sinter or actually fuse thecarbonaceous particles together. In another aspect of the presentinvention, an interstitial material may be selected which chemicallybonds the packed diamond particles together.

The choice of interstitial material must account for the thermalconductivity and heat capacity of the interstitial material itself. Adiamond compact heat spreader which contains material having a lowthermal conductivity will act as a limiting element within the structurethus obviating some of the heat transfer benefits of diamond. Therefore,an interstitial material which has high thermal conductivity, low heatcapacity, and provides for a chemical bond with diamond greatlyfacilitates the heat transfer across interface boundaries. Of course, alarger degree of diamond-diamond contact will also improve the heattransfer properties of the heat spreader.

The interstitial material for bonding or sintering of carbonaceousparticles may be provided in a number of ways including infiltration,sintering and electro-deposition. Infiltration occurs when a material isheated to its melting point and then flows as a liquid through theinterstitial voids between particles. Sintering occurs when theinterstitial material is heated sufficient to cause neighboringparticles of material to melt near their edges and sinter neighboringparticles together in an essentially solid-state process. Thus,substantially no fluid flow of the interstitial material would occur.Electro-deposition involves depositing a metal heated above its meltingpoint in solution on the surface of the carbonaceous particles under anelectrical current.

Two basic categories of interstitial material include liquid metal andmolten ceramics. When bonding the carbonaceous particles to produce acarbonaceous composite heat spreader the interstitial material shouldcontain at least one active element that will react with carbon to formcarbide. The presence of a carbide former aids in the wetting of thediamond particles and causes the interstitial material to be pulled intothe interstitial voids by capillary force. When sintering thecarbonaceous particles to produce a carbonaceous heat spreader, theinterstitial material should act as a sintering aid to increase thedegree of carbon sintering and does not necessarily contain a carbideformer but rather contains a carbon solvent.

In one aspect of the present invention, an interstitial infiltratingalloy can be used as an infiltrant to bond the carbonaceous particlesinto a substantially solid heat spreading mass. As mentioned above, manyinterstitial materials may actually hinder the transfer of heat throughthe heat spreader. For example, interstitial materials that do notchemically bond with carbon, but merely hold it mechanically can slowdown the transfer of heat. Further, many refractory materials that aregood carbide formers are poor heat conductors.

An additional consideration, when the carbonaceous material is diamond,is that care must be taken in choosing an interstitial material so as toavoid an infiltration or sintering temperature that is high enough todamage the diamond. Therefore, in one aspect of the invention, theinterstitial material may be an alloy that melts or sinters below about1,100° C. When heating above this temperature, the time should beminimized to avoid excessive damage to the diamond particles. Damage tothe diamond particles may also be induced internally due to cracking ofthe diamond from the site of metal inclusions. Synthetic diamonds alwayscontain a metal catalyst (e.g., Fe, Co, Ni or its alloy) as inclusions.These metal inclusions have high thermal expansion coefficients and theycan back-convert diamond into graphitic carbon. Hence, at hightemperature, diamond will crack due to the different thermal expansionof metal inclusions or back-convert diamond to carbon.

In accordance with the present invention, interstitial materials maycontain a diamond or carbon braze as a metal infiltrant or siliconalloys as ceramic infiltrants. Moreover, the infiltrant may be able to“wet” carbon so it can be wicked in the interstitial of carbonaceousparticles by capillary force. The interstitial material substantiallyfills any of the remaining voids between the packed carbonaceousparticles. Common carbon wetting agents include Co, Ni, Fe, Si, Mn, andCr. When the carbonaceous particles are to be chemically bonded togetherthe interstitial material may contain a carbide former which providesfor improved thermal properties at the boundaries between particles.Such carbide formers include Sc, Y, Ti, Zr, Hf, V, Nb, Cr, Mo, Mn, Ta,W, Tc, Si, B, Al, and alloys thereof.

Interstitial or infiltrating materials of the present invention mayinclude a component such as Ag, Cu, Al, Si, Fe, Ni, Co, Mn, W, or theiralloys or mixtures. Diamond or carbon brazes include Fe, Co, or Nialloys which exhibit wetting of the carbonaceous particles. Alloys ofthese brazes may also contain a carbide former such as Ti, Zr, or Cr.Ceramic silicon alloys may contain Ni, Ti, or Cr. For example, Ni—Cralloys, such as BNi2 (Ni—Cr—B) or BNi7 (Ni—Cr—P) are good infiltrants.Other examples of effective infiltrants include Al—Si, Cu—Sn—Ti,Ag—Cu—Ti, and Cu—Ni—Zr—Ti. Most carbonaceous interstitial materialscontain active elements (e.g., Cr, Ti) that not only bond to carbon byforming carbide, but are also easily oxidized. Hence, the introductionof interstitial materials should be performed in a vacuum furnace orunder the protection of an inert atmosphere.

The above carbonaceous composite heat spreaders can be produced by atleast partially filling in the pores or gaps among carbonaceousparticles by an interstitial material that can conduct heat relativelyfast. The interstitial material may be introduced into the packedparticles in a variety of ways. One way to provide the interstitialmaterial is by electro-deposition (e.g., Ag, Cu, Ni) in a watersolution. The metal is most often provided in an acid solution and maybe performed by those skilled in this art. Various additional elementsmay also be added to lessen the surface tension of the solution or tootherwise improve infiltration into the voids.

In another aspect of the invention, a carbonaceous heat spreader isprovided that includes a plurality of carbonaceous particles present inan amount of at least about 50% by volume of the heat spreader. Anon-carbonaceous infiltrant is present in an amount of at least about 5%by volume of the heat spreader. The non-carbonaceous infiltrant caninclude an element selected from the group consisting of Cu, Al and Ag.In a further aspect of this embodiment, the carbonaceous particles canbe present in an amount of at least about 80% by volume of the heatspreader, or at least 90% by volume of the heat spreader.

As in other aspects of the invention, the carbonaceous particles cancomprise diamond particles. The diamond particles can be present in anamount of greater than about 50% by volume of the heat spreader. Theinfiltrant, which can contain at least about 2% w/w of a carbide former,can be present in an amount of greater than about 5% by volume of theheat spreader.

In another aspect of the invention, a heat spreader is provided thatincludes carbonaceous particles and a non-carbonaceous infiltrant thatmay or may not chemically bond to the carbonaceous particles. In thisembodiment, the non-carbonaceous infiltrant can be, for example, Cu, Alor Ag. By processing the heat spreader under relatively high pressures,the non-carbonaceous infiltrant increase the resulting heat spreadingcapacity of the heat spreader while increasing the packing density ofthe carbonaceous particles. For instance, in the case where thecarbonaceous particles are diamond, when the heat spreader is processedat ultrahigh pressure the diamond grains can at least partially crush atdiamond-to-diamond contact points. As the diamond particles crush,molten Cu, Al or Ag can be partially injected into the diamond grainsand thereby lead to an increase in the diamond particle density.

As the resulting heat spreader includes a high concentration of diamondparticles, with substantially all of the voids between diamond particlesbeing filled with Cu, Al, or Ag, the heat spreader can exhibit a highdegree of thermal conductivity. Heat spreaders made in accordance withthis embodiment have been found to exhibit thermal conductivity on theorder of 1½ to 2 times that of pure copper. In addition, as Cu, Al andAg are relatively inexpensive materials, a heat spreader in accordancewith this embodiment can be made at commercially competitive costs.Also, as Cu, in particular, has relatively low melting point (belowabout 1100° C.), the process can be conducted at lower temperatures andpressure that may otherwise be required in conventional diamond PCDformation processes.

Some prior art diamond composite heat spreaders have been formed usingdiamond particles coated with a carbide or carbide forming material. Incontrast, in one aspect of the present invention, the diamond particlesand Cu, Al or Ag can each be used in their base, untreated form. Thiscan eliminate the costly process of coating diamond particles prior toinfiltrating a mass of the particles with an infiltrant. In addition,the diamond particles used in heat spreaders in accordance with thepresent invention can utilize diamond particles having a relativelycourse grain, for example grains of 50 microns and larger. This canresult in fewer grain boundaries present to slow down heat flow.

Another way to provide the interstitial material is by sintering of asolid powder in the voids between carbonaceous particles. Sintering maybe accomplished in a variety of ways, known to those skilled in the artsuch as, but not limited to, hot pressing, pressure-less sintering,vacuum sintering, and microwave sintering. Although hot pressing is acommon method, microwave sintering is becoming an increasingly usefulmethod as it allows for faster sintering times and decreased porosity.This is particularly advantageous in the present invention because themicrowave acts to primarily heat the sinterable metal material ratherthan the carbon. When diamond is used, this helps to reduce degradationof the diamond during processing.

A sinterable interstitial material may be provided during the packingprocess, in which case the sintered material occupies much of the spacebetween carbonaceous particles and prevents substantialparticle-particle contact. However, the sinterable interstitial materialmay be introduced in a similar manner to that used in successive packingof smaller diamond particles, wherein the size of the interstitialmaterial is chosen so as to allow the material to partially fill thevoids between carbonaceous particles after the carbonaceous particleshave been packed. Once the voids are sufficiently filled theinterstitial material is sintered. In this manner the particle-particlecontact can be improved.

A third way to provide the interstitial material is to infiltratediamond particles with a molten material (e.g., Al, Si, BNi2). Theelectro-deposited metal cannot bond carbon chemically so carbonaceousparticles are entrapped inside. Further, the sintered material may nothold particles firmly because bonding to carbonaceous particles duringsintering is primarily mechanical. The infiltrant should contain anactive element so it can react with carbon to form chemical bonds in theform of carbide. The presence of a carbide former also allows theinfiltrant to wet the particle surface and draw the infiltrant furtherinto the interstitial voids by capillary action.

When diamond particles are used, in order to minimize the diamonddegradation, the infiltration is preferably performed at a temperaturebelow 1,100° C. Many of the Fe, Ni, and Co alloys mentioned above havemelting temperatures in this range. During infiltration or sintering ofan interstitial material, the hot metal will inevitably cause some smalldegree of diamond degradation. However, this effect may be minimized byreducing the processing time and carefully choosing the interstitialmaterial. Silicon is particularly good at filling the interstitial voidsbetween diamond particles due to its tendency to form SiC by reaction.The formation of SiC at the interface between diamond and molten Si mayprotect diamond from further deterioration. The melting temperature ofpure Si is approximately 1,400° C. Under a high vacuum (e.g. below about10⁻³ such as 10⁻⁵ torr), molten Si or its alloy can infiltrate intodiamond effectively without excessively damaging diamond so a good heatspreader can be fabricated.

Thus, the interstitial material may be introduced into the packedcarbonaceous particles by infiltration, sintering or electro-deposition.When performed at low pressures, these interstitial materials merelyfill the voids between particles and bond the particles together. Atvery high pressures there are two basic possibilities. First, theinterstitial material may chemically bond with the carbon and/or providebeneficial thermal properties across the carbonaceous material tointerstitial material interface and the carbonaceous material will bepartially crushed to eliminate a portion of the voids. Second, if theinterstitial material is a carbon solvent such as, but not limited to,iron, cobalt, nickel or alloys of these materials, the carbonaceousparticles will sinter together to form a continuous carbonaceous mass.When the carbonaceous particles sinter together, the path for heattransfer is essentially a continuous carbon path having substantially nomechanical or non-carbon interfaces to traverse.

In one embodiment of the present invention, copper is used as theinterstitial material. Copper is an ideal thermal conductor for makingdiamond heat spreaders. However, copper is not a carbon solvent and isnot a catalyst for graphite to diamond conversion, nor does it act as asintering aid at ultrahigh pressure. Hence, if copper is used as theinterstitial material, it can also be done by electro-deposition orsintering. However, electro-deposition is extremely slow and inefficientin filling the pores among tightly packed diamond grains. Sintering, onthe other hand, will inevitably leave copper caught between diamondgrains. In either method, the carbonaceous particle packing efficiencyin the final heat spreader is relatively low (e.g., 60% by volume).

Although copper is not a sintering aid to sinter carbonaceous particlestogether along carbonaceous grain boundaries, the ultrahigh pressureconsolidation of a carbon-copper mixture can force carbonaceous grainscloser together to reach a higher carbon content such as 70% by volume.Pressures may range from about 4 GPa to about 6 GPa. At these highpressures some of the carbonaceous particles are partially crushed toeliminate a portion of the voids between particles. In order to attainover 70% by volume of carbon without forming carbon-to-carbon bridgesthe excess copper must be extracted by a sink material. This sinkmaterial contains pores under ultrahigh pressure conditions and wouldnot soften at the melting temperature of copper. Such a sink materialmay be made of a ceramic powder such as SiC, Si3N4, and Al2O3, but mayalso be formed of any porous material which provides a sufficient mediumfor absorbing the excess copper. Other useful porous materials includeWC and ZrO2. This technique may be further explained by reference toExample 1 below.

In another aspect of the present invention, the interstitial materialmay be a carbon wetting infiltrating alloy with low percentage contentby volume of the carbon wetting agent. In this manner, mechanicalinterfaces between carbon and the infiltrant are greatly reduced whileproviding an infiltrant with a relatively high thermal conductivity. Forexample, a good heat conducting metal such as Ag, Cu, or Al, may bealloyed with a carbide former such as Ti.

In addition to the benefits provided by including an infiltrating alloywith good conductivity and diamond wetting properties, the infiltratingalloy can be selected such that it has a relatively low eutectic meltingpoint. In this manner the above-recited disadvantages associated withprocessing diamond particles at extremely high temperatures andpressures can be avoided. In one aspect, the eutectic melting point ofthe alloy used may be less than about 1100° C. In another aspect, themelting point may be less than about 900° C.

Examples of good carbide forming elements include without limitation,those recited above. Further, examples of materials having high heatconductivity include without limitation, Ag, Cu, and Al. A wide range ofspecific alloys may be used which attain the desired heat transfer andchemical bonding properties and also have a eutectic melting pointwithin the temperatures specified above. However, in one aspect of theinvention, the infiltrating alloy can include a carbide forming elementand at least 1 wt % to about 10 wt % of either Ag, Cu, or Al. In anotheraspect, the carbide forming element is present in an amount of at leastabout 1% w/w of the heat spreader.

In another aspect, the infiltrating alloy can comprise a Cu—Mn alloy.The Cu—Mn alloy can be Cu—Mn(30%)-Ni(5%), which has a melting point ofabout 850° C., much lower than sintering temperatures used in the past.For example, Co, which has a melting point of about 1500° C., is oftenused as a sintering aid in HPHT processes used to form PCD compacts. Aspreviously noted such temperatures endanger the integrity of thecarbonaceous material and can cause degradation thereof. This isespecially true for diamond particles. In contrast, processing diamondparticles at relatively low temperatures of about 850° C. is much moredesirable, from both an integrity standpoint and a process control andcost standpoint. In addition to those materials recited above, theinfiltrating alloy used in the present invention can include a number ofother materials. For example, in one aspect of the invention,CuAlZr(9%)and CuZr(1%) can be used. While Zr is not a particularly goodthermal conductor, its presence in the infiltrating alloy is relativelysmall by volume, and therefore does not significantly inhibit heatconduction through the heat spreader.

By utilizing an infiltrating alloy which has a relatively low meltingpoint and also has good wetting and conductivity properties, a superiorcarbonaceous heat spreader can be provided. The lower operatingtemperature results in lower operating pressures. By requiring loweroperating pressures, the present invention can be utilized to formcarbonaceous heat spreaders with far greater thickness than prior artmethods, as it has been the extremely high pressures required to formPCDs that has limited the mold size used in the past. For instance, inone aspect, the present invention can be utilized to form heat spreaderswith a thickness greater than about 1 mm. In another aspect, heatspreaders made in accordance with the present invention can havethickness as high as and exceeding about 2 mm. By forming the heatspreader with a greater thickness, the resulting heat spreader has thecapacity to transfer or spread a greater volume of heat per unit timeand therefore has a significantly greater cooling capacity.

Heat spreaders made in accordance with the present invention may take avariety of configurations based on the intended use. The carbonaceousmaterial made as described above may be polished and shaped based on theparticular requirements of the heat source to which it will be applied.In contrast to CVD, the carbonaceous composites herein can be formed toalmost any size relatively quickly. Most often for electronicapplications the heat spreader will be between about 0.1 mm and about 1mm thick. The heat spreader may be formed into a circular or ellipticaldisk or a quadrilateral such as a square, rectangular or other shapedwafer. The heat source may be any electrical or other component whichproduces heat.

Once the heat spreader is formed, appropriate placement is based ondesign and heat transfer principles. The heat spreader may be in directintimate contact with the component, and may even be formed to encompassor otherwise be contoured to provide direct contact with the heat sourceover a wide surface area. Alternatively, the heat spreader may beremoved from the heat source by a heat conduit or other heat transferdevice.

In addition to the heat spreader disclosed herein, the present inventionencompasses a cooling unit for transferring heat away from a heatsource. As shown in FIG. 1 a, a heat spreader 12, formed in accordancewith the principles discussed herein, can be disposed in thermalcommunication with both a heat source, such as a CPU 14, and a heat sink16. The heat spreader transfers heat created by the CPU to the heatsink. The heat sink can be a number of heat sinks known to those ofordinary skill in the art including both the materials andconfigurations thereof. For example, aluminum and copper are well knownfor use as heat sinks, and as shown in FIG. 1, can have a configurationthat includes cooling fins 18. As heat is quickly and efficientlytransferred from the CPU through the heat spreader, the heat sinkabsorbs the heat, and the cooling fins help dissipate the heat into thesurrounding environment. A number of contact configurations between theheat sink, heat source, and heat spreader can be utilized depending onthe specific results to be achieved. For example, the components may bedisposed adjacent each other and can also be bonded or otherwise coupledto each other. In one aspect of the invention, the heat spreader can bebrazed to the heat sink.

While the heat sink 18 is shown in the figures as a sink includingcooling fins, it is to be understood that the present invention can beutilized with any heat sink known to those in the art. Examples of knownheat sinks are discussed in U.S. Pat. No. 6,538,892, which is hereinincorporated by reference. In one aspect of the invention, the heat sinkcomprises a heat pipe having an internal working fluid. Examples of heatpipe heat sinks are discussed in U.S. Pat. No. 6,517,221, which isherein incorporated by reference.

As shown in FIG. 1 b, in one aspect of the invention, the heat spreader12 can be at least partially embedded in the heat sink and/or the heatsource. In this manner, not only is heat transferred from a bottom ofthe heat spreader to the heat sink, but heat is also at least partiallytransferred from sides of the heat spreader into the heat sink. Afterbeing embedded in the heat sink, the heat spreader can be bonded orbrazed to the heat sink. In one aspect, the heat spreader can be held inthe heat sink by a compression fit. In this manner, no bonding orbrazing material exists between the heat spreader and the heat sink,which might act as a barrier to efficient heat transfer from thespreader to the sink.

While the heat spreader can be held in the heat sink by a variety ofmechanisms known to those skilled in the art, in one aspect the heatspreader is held in the heat sink by a thermally induced compressionfit. In this embodiment, the heat sink can be heated to an elevatedtemperature to expand an opening formed in the heat sink. The heatspreader can then be fitted into the expanded opening and the heat sinkcan be allowed to cool. Upon cooling, the heat sink, which has arelatively high coefficient of thermal expansion, will contract aroundthe heat spreader and create a thermally induced compression fit thatholds the heat spreader embedded within the heat sink without requiringany intervening bonding material. A mechanical friction fit can also beutilized to hold the heat spreader in the heat sink.

As shown in FIG. 1 c, in one aspect of the invention, the heat sink cancomprise a heat pipe 22 which can have an internal working fluid (notshown). The internal working fluid can be any known to those in the art,and in one aspect is water or water vapor. The heat pipe can besubstantially sealed to maintain the working fluid within the heat pipe.The heat spreader can be disposed adjacent the heat pipe and in oneaspect is brazed to the heat pipe. In the embodiment shown in FIG. 1 c,the heat spreader protrudes through a wall of the heat pipe so that abottom of the heat spreader is in direct contact with the working fluid.The heat spreader can be brazed within the heat pipe, as shown at 26, toassist in maintaining the substantially sealed condition of the heatpipe.

As the heat spreader is in direct contact with the working fluid, theworking fluid can more efficiently transfer heat from the heat spreader.In the embodiment shown in FIG. 1 c, the working fluid, in this casewater (not shown), contacts the heat spreader and becomes vaporized asit absorbs heat from the heat spreader. The water vapor can thencondense in liquid form on the bottom of the heat pipe, after which, dueto capillary forces, the liquid will migrate 24 back up the walls of theheat pipe to the heat spreader, where it will again vaporize and repeatthe cycle. As the walls of the heat pipe can be made of a material witha high coefficient of thermal conductivity, heat is dissipated from thewalls of the heat pipe into the surrounding atmosphere.

As mentioned above, the packed carbonaceous particles, especiallydiamond, may also be sintered together to form a mass of substantiallysintered particles having largely only carbon. When the carbonaceousparticles are sintered together there are carbon bridges connectingneighboring carbon particles. The above-described packing methods canincrease the original carbon packing efficiency. By packing differentsize carbonaceous particles in successive stages the packing efficiencymay be increased up to about 80% by volume. However, because there is nocarbon-to-carbon bonding, the packing efficiency reaches a limit. Hence,in order to further increase the packing efficiency and the thermalconductivity, carbonaceous particles must be sintered together. Inaddition, when the carbonaceous particles are sintered together suchthat there are carbon bridges connecting neighboring carbonaceousparticles an uninterrupted path for heat flow is provided. In this way,heat can pass through the carbonaceous heat spreader rapidly withoutbeing slowed down at interfaces between individual particles which aremerely in intimate contact.

In order for diamond particles to sinter together, they must be heatedin the stability region of diamond, otherwise, diamond will revert tothe more stable form of graphite. U.S. Pat. Nos. 3,574,580; 3,913,280;4,231,195 and 4,948,388 discuss this process in more detail and are allincorporated herein by reference. Diamond sintering is generallyperformed at very high pressures. Typically, pressures of more thanabout 4 GPa up to about 8 GPa are required, although a few processeshave sought to lower this pressure requirement, e.g., U.S. Pat. No.4,231,195. More typical sintering pressure is about 5 to about 6 GPa. Atsuch pressures, diamond particles sinter together by a mechanism knownas liquid phase sintering.

An interstitial material may be provided which acts as a diamond, orcarbonaceous particle, sintering aid. During this process, aninterstitial material (e.g., Fe, Co, Ni, Si, Mn, Cr) can wet the diamondparticles. The diamond will dissolve into this interstitial materialbecause of increased solubility at these pressures. The local pressureis higher at the contact points of the diamond particles, so diamondparticles will dissolve first at these points. In contrast, the pressurein the original voids between diamond particles is low so the dissolveddiamond in the form of carbon atoms in the molten liquid willprecipitate out as diamond in the voids. Hence, the contacting points ofdiamond will gradually dissolve and the voids between the diamondparticles will gradually fill with precipitated diamond. The consequenceis to bring diamond particles closer beyond the original contact pointand the substantial elimination of the original voids to produce adiamond structure having a composition between about 70% and about 98%by volume of diamond. In addition, unlike with the low-pressureprocesses described above the diamond particles will not experience anydegradation because the conditions of temperature and pressure arewithin the stability region of diamond.

The final product of ultrahigh pressure sintering of diamond is apolycrystalline diamond (PCD) with remnant diamond grains sinteredtogether. In such a structure, the outlines of the original diamondparticles are largely lost and instead prominent diamond-to-diamondbridges are formed. If diamond sintering can be performed nearcompletion, the entire mass will be made of diamond with small pocketsof non-diamond material trapped in the original voids inside the PCD.Such a structure may contain over 95% by volume of a continuousframework of diamond and hence it is highly efficient in conducting heatand approaches the thermal properties of pure diamond.

This ultrahigh pressure process may also be applied to carbonaceouscomposite heat spreaders made by sintering of metal together at a lowerpressure (<2 GPa) as in the case of hot pressing mentioned above. Theultrahigh pressure process may also be used to consolidate carbonaceouscomposite heat spreaders to increase the carbon content beyond what canbe achieved by hot pressing alone.

Interstitial materials suitable for the ultrahigh pressure production ofheat spreaders according to the method of the present invention includeSi, Ti, Fe, Co, Ni, Cu, Mn, W, La, Ce, and mixture or alloys of thesematerials. Not all of these materials act as a sintering aid.

In another aspect of the present invention, a sink material such as aceramic is provided to accelerate the removal of the sintering aids. Asdescribed above, this sink material is porous and does not soften at theultrahigh pressures used in sintering of the diamond particles. Suchsink materials are most often ceramic powders such as SiC, Si3N4, andAl₂O₃, but may be any porous medium which can act to absorb excesssintering aid material. Other useful porous materials include WC andZrO2.

In addition to the above-recited systems and devices, the presentinvention also provides a method for making a heat spreader, and caninclude the steps of providing a plurality of carbonaceous particles,and infiltrating the plurality of carbonaceous particles with anon-carbonaceous infiltrant as recited in the various aspects describedabove, such that a heat conducting mass is formed. In another aspect, amethod of cooling a heat source is provided and includes the steps ofproviding a heat spreader as recited in the various aspects describedabove, and placing the heat spreader in thermal communication with boththe heat source and a heat sink.

The following examples present various methods for making the heatspreaders of the present invention. Such examples are illustrative only,and no limitation on the present invention is meant thereby.

EXAMPLES Example 1

Diamond particles can be mixed with powdered copper to form a mixture.This mixture is then cold pressed to form a slug. A thin walled moldmade of a refractory metal (e.g., Ti, Zr, W, Mo, Ta) is provided.Ceramic particles (e.g., SiC, Si₃N₄, Al₂O₃) having a coarse grain size(e.g., 40/50 mesh) are first put in the mold and then the ceramicparticles are covered with the diamond-copper slug. The sample assemblyis then placed in a high-pressure cell and pressurize to over 5GPa. Theassembly is then heat charged to over 1200° C. by passing an electriccurrent through a heating tube that surrounds the sample assembly. Atthis temperature and pressure, copper melts and is forced out frombetween the diamond particles. The liquid copper flows to the bottom ofthe mold containing the ceramic particles. The ceramic particles containample empty pores to receive the liquid copper. In this way the diamondgrains are partially crushed and substantially fill in the space left bythe copper. The result is a high diamond content (e.g., 85% by volume)heat spreader. A portion of the copper remains in the composite materialand is bonded to the diamond to hold the particles together.

Because of the lack of diamond-to-diamond bridges, the copper cementeddiamond composite described above does not reach a diamond content of upto 95% by volume of sintered diamond as in PCD, but its diamond contentis much higher than would be produced by electro-deposition orhot-pressing. Hence, the thermal conductivity would be much higher thanthe low-pressure diamond composite heat spreaders of the presentinvention. Moreover, the high thermal conductivity of copper partiallycompensates for the lower diamond content (about 80% by volume) whencompared to PCD as the latter contains carbon solvent metals, e.g., Co,that have a lower thermal conductivity than copper.

PCD has been made routinely, but is typically designed and usedexclusively for mechanical functions, such as cutting tools, drill bits,and wire drawing dies. In order to improve the mechanical finish and toincrease the mechanical strength (e.g., impact strength), PCD is made ofvery fine diamond powder. The best PCD contains very fine diamondparticles such as sub-micrometer sizes (e.g., manufactured by SumitomoElectric Company of Japan). By utilizing PCD in a heat spreader,mechanical properties become less important. Instead of impact strengthand surface finish, the diamond packing efficiency and thermalproperties are the primary concern. Thus, the design of PCD for heatspreaders is distinct from that of conventional abrasive applications.Specifically, the diamond particles of the present invention arerelatively large grain sizes, and the infiltrant or sintering aidrequires high thermal conductivity rather than mechanical toughness asin conventional PCD.

In order to improve the heat transfer efficiency of the heat spreaderthe grain boundaries of diamond particles are minimized, this is incontrast to a conventional design of diamond composites where the grainboundaries are maximized. The use of larger diamond particles not onlyreduces the grain boundaries that reduce heat transfer, but also servesto increase the diamond packing efficiency and further increase thethermal conductivity. Hence, this design criterion is applicable to alldiamond and diamond composite heat spreaders described herein.

Example 2

A contact cavity in the form of a circular hole having a diameter ofabout 20 mm is formed into a flat base of an aluminum heat sink withradiating fins cooled by a fan. The heat sink is heated to a temperatureof about 200° C. to expand the contact cavity, after which a diamondcomposite heat spreader with a diameter of about 20 mm is inserted intothe contact cavity. Upon cooling, the much larger thermal expansioncontraction of the aluminum heat sink will result in the diamondcomposite heat spreader being firmly compressed into the contact cavity.The top surface of the diamond composite heat spreader is ground toremove any debris formed by the shrink fitting. The heat spreader isplaced in contact with a chip or CPU and heat is spread through the heatspreader and into the heat sink, with heat spreading from both thebottom of the heat spreader and the sides of the heat spreader into theheat sink.

Example 3

50/60 U.S. mesh diamond particles are acid cleaned and loaded in atatalum cup having a cylindrical shape. An oxygen-free copper disk isplaced on top of the diamond particles. The charge is pressurized to 5.5GPa in a 2000 ton cubic press that utilizes 6 anvils pressing toward apyrophllite cube that contained the charge. Electrical current is passedthrough a graphite tube that surrounds the charge. At a temperature of1150 C, molten copper is infiltrated through the diamond particles. Uponcooling and decompression, the charge is ground to remove the tataliumcontainer and also the top and bottom surfaces of the diamond-coppercomposite. The final disk is 37 mm in diameter and 2 mm thick. Thediamond content is approximately 82 V %. The resulting diamond-copperheat spreader has a heat transfer rate of about 1.5-2 times that of purecopper.

Example 4

A heat spreader is made in accordance with Example 3, except that Cu—Zrin concentration of about 1 wt % is used to improve the wettingcharacteristics of copper and diamond.

Example 5

Grafoil is placed in an alumina container and is covered with 30/40diamond crystals. The crystals are pressed into the grafoil by using aflat plate. AgCuSnTi foil is placed on top of diamond/grafoil mixture.The assembly is heated in a vacuum furnace at 950 C for 15 minutes. Theresult is an alloy infiltrated diamond-graphite.

Example 6

30/40 mesh diamond particles (about 500 micrometers) are mixed withbronze powder (about 20 micrometers) to achieve a volume efficiency of50%. The mixture is hot pressed in a graphite mold to a pressure of 40MPa (400 atmospheric pressure) and heated to 750° C. for 10 minutes. Theresult is a diamond metal composite disk of 30 mm in diameter and 3 mmin thickness.

Example 7

30/40 mesh diamond particles are mixed with aluminum powder and loadedin an alumina tray. The charge is heated in a vacuum furnace of 10⁻⁵torr to 700° C. for 5 minutes so the aluminum becomes molten. Aftercooling, result is a diamond aluminum composite.

Example 8

30/40 mesh diamond is placed inside a graphite mold and covered withNICROBRAZ LM (Wall Colmonoy) powder of about 325 mesh. The load isheated in a vacuum furnace of 10⁻⁵ torr to 1010° C. for 12 minutes. Themolten Ni—Cr alloy infiltrated into diamond particles to form a diamondmetal composite.

Example 9

30/40 mesh diamond is placed inside a graphite mold and covered withbroken silicon wafers. The load is heated in a vacuum furnace of 10⁻⁵torr to 1470° C. for 9 minutes. The molten Si infiltrated into diamondparticles to form a composite.

Example 10

30/40 mesh diamond is placed inside a graphite mold and then agitated.220/230 mesh diamond is then placed in the mold and gently agitateduntil most of the voids are filled with the smaller particles. Thepacked diamond is then covered with NICROBRAZ LM (Wall Colmonoy) powderof −325 mesh. The load is heated in a vacuum furnace of 10⁻⁵ torr to1,010° C. for 12 minutes. The molten Ni—Cr alloy infiltrated intodiamond particles to form a diamond metal composite.

Example 11

30/40 mesh diamond is packed around a cathode and immersed in an acidbath that contains copper ions. After the current passes through, copperis gradually deposited in the pores of these diamond particles. Theresult is a diamond copper composite.

Example 12

20/25 mesh diamond particles (SDA-100S made by De Beers) substantiallycubic in shape were aligned edge to edge on an alumina plate to form asingle layer of diamond particles about 40 mm square. A silicon wafer of0.7 mm in thickness was placed on top of this layer of particles. Theassembly was then placed in a vacuum furnace and pumped down to 10⁻⁵torr. The temperature was then raised to 1,450° C. for 15 minutes. Thesilicon melted and infiltrated between the diamond particles. Aftercooling, the composite was machined to eliminate excess silicon. Theresult is a diamond heat spreader of about 0.8 mm. This heat spreadercontains a diamond volume of about 90%. The use of substantially cubicparticles allows a much higher diamond content than can beconventionally achieved using the successive packing method describedearlier.

Example 13

40/50 mesh diamond particles are mixed with a mixture of Si and Tipowders and the entire mixture is loaded inside a graphite mold that isin turn fitted inside a titanium heating tube. The assembly is placed atthe center of a pyrophyllite block. This block is mounted in a cubicpress and it is subjected to a pressure of 5.5 GPa. Heating is achievedby passing electrical current through the titanium tube. When thesilicon melts it dissolves titanium and both flow around the diamondparticles. Diamond particles then sinter with the aid of the siliconliquid. After quenching and decompression, the diamond composite isseparated from the pyrophillite and other pressure medium. The result isa diamond composite that contains about 92% by volume of diamond. Twentysuch diamond composites are made each with dimensions of 20 mm indiameter and 3 mm in thickness. These diamond composite disks werepolished by diamond wheels and measured for thermal conductivity thatindicates a value of about twice that of copper.

Of course, it is to be understood that the above-described arrangementsare only illustrative of the application of the principles of thepresent invention. Numerous modifications and alternative arrangementsmay be devised by those skilled in the art without departing from thespirit and scope of the present invention and the appended claims areintended to cover such modifications and arrangements. Thus, while thepresent invention has been described above with particularity and detailin connection with what is presently deemed to be the most practical andpreferred embodiments of the invention, it will be apparent to those ofordinary skill in the art that numerous modifications, including, butnot limited to, variations in size, materials, shape, form, function andmanner of operation, assembly and use may be made without departing fromthe principles and concepts set forth herein.

1. A heat spreader comprising: a plurality of carbonaceous particlespresent in an amount of at least about 50% by volume of the heatspreader; and a non-carbonaceous material present in an amount of atleast about 5% by volume of the heat spreader, said non-carbonaceousmaterial including an element selected from the group consisting of Cu,Al and Ag.
 2. The heat spreader of claim 1, wherein the carbonaceousparticles are present in an amount of at least about 80% by volume ofthe heat spreader.
 3. The heat spreader of claim 2, wherein thecarbonaceous particles are present in an amount of at least about 90% byvolume of the heat spreader.
 4. The heat spreader of claim 1, whereinthe carbonaceous particles are diamond particles.
 5. The heat spreaderof claim 1, wherein the non-carbonaceous material consists essentiallyof either Cu, Al, or Ag.
 6. The heat spreader of claim 1, wherein thenon-carbonaceous material includes a carbide forming element from about1% w/w to about 10% w/w of the non-carbonaceous material.
 7. The heatspreader of claim 6, wherein the carbide forming element is present inan amount of at least about 1% w/w.
 8. The heat spreader of claim 7,wherein the carbide forming element is a member selected from the groupconsisting of: Sc, Y, Ti, Zr, Hf, V, Nb, Cr, Mo, Mn, Ta, W, Tc, Si, B,Al, and alloys thereof.
 9. The heat spreader of claim 8, wherein thecarbide forming element is a Cu—Mn alloy.
 10. The heat spreader of claim1, wherein the non-carbonaceous material is an alloy having a eutecticmelting point below about 1100° C.
 11. The heat spreader of claim 1,wherein the non-carbonaceous material is an alloy that wets thecarbonaceous particles.
 12. The heat spreader of claim 4, wherein thenon-carbonaceous material contains at least about 2% w/w of a carbideformer.
 13. The heat spreader of claim 1, wherein the heat spreader hasa thickness of greater than about 1 millimeter.
 14. A cooling unit fortransferring heat away from a heat source, comprising: a heat sink; anda heat spreader as recited in claim 1 disposed in thermal communicationwith both the heat sink and the heat source.
 15. A cooling unit fortransferring heat away from a heat source, comprising: a heat sink; anda heat spreader comprising a mass of diamond particles sintered directlyto one another in thermal communication with both the heat sink and theheat source.
 16. The cooling unit of either of claims 14 or 15, whereinthe heat spreader is at least partially embedded in the heat source. 17.The cooling unit of either of claims 14 or 15, wherein the heat spreaderis at least partially embedded in the heat sink.
 18. The cooling unit ofclaim 17, wherein the heat spreader is held in the heat sink by acompression fit.
 19. The cooling unit of claim 18, wherein thecompression fit holding the heat spreader in the heat sink is athermally induced compression fit.
 20. The cooling unit of either ofclaims 14 or 15, wherein the heat spreader is brazed to the heat sink.21. The cooling unit of either of claims 14 or 15, wherein the heat sinkcomprises a heat pipe having an internal working fluid.
 22. The coolingunit of claim 21, wherein the heat spreader protrudes through a wall ofthe heat pipe, and has a bottom surface in direct contact with theworking fluid of the heat pipe.
 23. A method of making a heat spreadercomprising the steps of: providing a plurality of carbonaceousparticles; and infiltrating the plurality of carbonaceous particles witha non-carbonaceous material as recited in any of claims 1 or 5-11, suchthat a heat conducting mass is formed.
 24. A method of cooling a heatsource comprising the steps of: providing a heat spreader as recited inany of claims 1-13; and placing the heat spreader in thermalcommunication with both the heat source and a heat sink.
 25. The heatspreader of claim 4, wherein the diamond particles are un-coated diamondparticles.